A complementary metal oxide semiconductor (CMOS) image sensor is a key component of many digital video cameras. The CMOS image sensor is typically comprised of an upper stack that includes one or more layers of color filters, a microlens array, and an overcoat for the microlens and a lower stack that includes interlevel dielectric (ILD) layers, interlevel metal (ILM) layers, and passivation layers which are fabricated on a substrate. High sensitivity is an important characteristic for a CMOS image sensor since the image quality may suffer if sensitivity is not high enough. The function of the microlens component is to focus light through a color filter layer and the lower stack onto the sensing area or photodiode. Other layers in the light path must have a high transparency to that a minimal amount of light intensity is lost.
The elementary unit of the image sensor is a pixel which is an addressable area element with intensity and color attributes related in large part to the spectral signal contrast obtained from the photon collection efficiency of the microlens array, spectra transmission through the color filters, microlenses, and other layers in the imaging path, and the spectral response and efficiency of the photodiode. Constant advances in technology that have reduced the smallest dimension in the CMOS device to less than a micron have also forced the pixel size to shrink to less than 5 um. When a plurality of color regions are formed in a color filter layer, the width of a color region is called a pixel. Newer technologies require an increased number of ILM layers that lengthens the distance (focal length) between a microlens and a photodiode. The longer focal length is a big challenge to maintaining adequate sensor sensitivity. Although a thinner microlens is able to produce a longer focal length, the quantum efficiency of a thin microlens is lower than that of a thicker microlens because of less surface area. Therefore, an improved design is needed for the upper stack and especially for the microlens component in a CMOS image sensor that increases sensitivity and is compatible with a pixel size that is smaller than 5 um.
A microlens is typically formed by patterning a photoresist that is preferably a positive tone type in which unexposed regions of the photoresist layer are chemically unaltered and remain on the substrate after an aqueous base developer is used to remove exposed regions. The resulting photoresist pattern is heated to a temperature that deforms a stripe or rectangular shape into a cylinder shape or a square shape into a hemisphere shape as shown in FIG. 1. The thermal treatment produces a microlens 1 with a flat bottom and top surface curvature such that the thickness T1 hereafter referred to as the radius of curvature in the center of the microlens 1 is greater than the thickness TX at other points (X) on the microlens surface. The thickness TX becomes progressively smaller as the distance D from the center increases until reaching a minimum at the edges 3. This design enables light 4 from above the microlens to be focused to a point 5 on a photodiode 6 below the microlens. The distance between the microlens 1 and the focal point 5 is the focal length FL. In this design, a spacer layer 7, color filter layer 8, planarization layer 9, ILD layer 10, and ILM layer 11, and protective layer 12 are also depicted. Although color filter layer 8 is shown as a single layer with regions 8a, 8b, 8c alternating between green 8a and red 8b or blue 8c, the color filters may also be stacked such that each color filter is contained in a separate layer. Furthermore, the color filter layer 8 may be located above the microlens 1 array rather than below it. The pixel width (PW) is shown as the width of one color filter region 8a, 8b, or 8c. 
U.S. Pat. No. 5,796,154 discloses a design including two layers of microlenses each having a thickness of about 2 microns that are separated by a transparent acrylate layer and preferably a color filter. The photorefractive index of the microlenses is higher than that of the acrylate and color filter. Since the lower microlens array does not reside on a planar surface, the size and oval shape of the microlens can be difficult to reproduce. In addition, the poly(vinylphenol)/diazonaphthoquinone diazide (DNQ) based photoresist that comprises the lower microlens has less suitable dissolution characteristics for patterning than a more common Novolak resin/DNQ photoresist.
A key feature in related U.S. Pat. Nos. 6,171,885 and 6,274,917 is microlens formation prior to color filter fabrication in order to minimize rework and place the microlens array in closer proximity to the photodiodes. A planarization layer is used to separate the microlens array from overlying color filter layers.
A high efficiency color filter process to improve color balance in an image sensor is described in U.S. Pat. No. 6,395,576. The color coating sequence involves coating a blue color filter layer first to form a color pixel structure with wider process window and improved adhesion to a substrate. The thickness of each color filter is adjusted in real time by a spectrophotometric algorithmic analyzer that drives a feedback servo control loop.